The error in the geographical position indicated by a satellite-based radio-navigation receiver depends on the errors which mar the measurements of pseudo-ranges determined by the algorithm used by this receiver and on the errors contained in the navigation message broadcast by the satellite.
In order to limit these errors and to ensure the protection of users, it is necessary to have means making it possible to identify such errors so that they can be eliminated and to calculate a bound on the position error as a function of the available measurements, while taking account of integrity and continuity specifications inherent to the context of use (landing of an aircraft, etc.). These means constitute the basis of a device having a RAIM (Receiver Autonomous Integrity Monitoring) function. Current GNSS receivers for civil aviation may not be used without a RAIM function.
Current equipment with a RAIM function suffers from two problems:                said equipment is completely integrated, thereby signifying that it is not possible to separately choose the item of equipment which acquires the navigation signal and the one which calculates the position of the item of equipment which provides the integrity functions,        said equipment is based on algorithms of least squares type which is destabilized by the presence of erroneous measurements whatever the amplitude (even infinitesimal) of the error which affects these measurements, thereby posing problems with the reliability of the position solutions proposed.        